Phase change technology is promising for next generation memory devices. It uses chalcogenide semiconductors for storing states and digital information. The chalcogenide semiconductors, also called phase change materials, have a crystalline state and an amorphous state. In the crystalline state, the phase change materials have low resistivity; while in the amorphous state, they have high resistivity. The resistivity ratios of the phase change materials in the amorphous and crystalline states are typically greater than 1000, and thus the phase change memory devices are unlikely to have errors for reading states. The chalcogenide semiconductors are stable at a certain temperature range in both crystalline and amorphous states and can be switched back and forth between the two states by electric pulses.
Typically, a phase change memory device is formed by placing a phase change material between two electrodes. Write operations, also called programming operations, which apply electric pulses to the memory device, and read operations, which measure the resistance of the phase change memory, are performed through the two electrodes. Generally, write operations utilize a set pulse and a reset pulse. The set pulse heats the phase change material to a temperature higher than a crystallization temperature Tx, but below a melting temperature Tm, for a time t2 longer than the required crystalline time, for the crystallization to take place. The reset pulse, which turns the phase change material into an amorphous state, heats the phase change material to a temperature higher than the melting temperature Tm. The temperature is then quickly dropped below the crystallization temperature Tx for a time period short enough to reduce or prevent the crystallization. The phase change material is heated by controlling the current flowing through a conductive material, commonly referred to as a “heater.” The heater comprises a conductive material that, due to its resistive properties, heats up when a sufficiently high voltage differential is applied.
One of the significant challenges that the phase change memory devices face is to reduce the programming current. A commonly used technique for reducing the programming current is to reduce the contact area between the phase change material and the heater. One attempt to reduce the contact area utilized a crown-type heater. Generally, a crown-type heater utilizes a heater formed along the sidewalls and bottom of a trench. This method, however, has a low area utility efficiency, making it difficult to scale down.
Another attempt to reduce the contact area utilizes a plug-type heater, wherein the heater comprises a plug or via formed through a dielectric layer. This attempt, however, may exhibit poor performance issues related to contact area disturbances as devices are scaled down.
Yet another attempt uses a line-type heater. In this attempt, the heater is formed by a damascene process in which a trench is formed and filled with a conductive material. The phase change material is then formed over the conductive material. In this attempt, however, it may be difficult to maintain uniformity as the design shrinks.
Accordingly, there is a need for a structure that provides reduced contact area while providing a high-level of uniformity and high contact density.